The present invention relates to vapor phase corrosion inhibiting compositions, and more particularly to inhibitors specifically formulated to provide corrosion protection of metal in recessed areas or encased, e.g. cables inside tubes.
Vapor phase corrosion inhibiting (VCI) materials are utilized in a variety of applications for protecting metal from corrosion, and generally include chemicals which function as corrosion inhibitors and which are primarily in the solid or liquid state at ambient temperatures, but which exhibit a small but significant vapor pressure. This volatility enables the corrosion inhibitors to migrate in the vapor phase to effectively protect proximate metal surfaces. Example vapor phase corrosion inhibitors are described in U.S. Pat. Nos. 2,752,221 and 4,275,835 herein incorporated by reference.
One prevalent application of VCI materials involves protecting metal in an enclosed space, such as electronics in a closed chassis or a metal article in a sealed package. In such situations, a vapor permeable packet containing VCI material can be inserted in the enclosure to provide corrosion protection to corrosion-susceptible items within the enclosure for an extended period of time (up to several years). However, experience has shown that there are limits to the above approach. In non-closed systems, the VCI can be lost to the outside atmosphere. Even in closed systems, the extent of corrosion protection tends to diminish at distances more than several feet from the VCI material packet. This is particularly problematic in enclosures with a high aspect ratio (e.g. inside a pipe). For this reason, a number of alternate delivery vehicles have been developed to extend the use VCI materials to a wider variety of applications. Example VCI delivery vehicles are described in U.S. Pat. Nos. 3,084,022, 5,715,945, 5,332,525, 6,028,160, and 6,555,600.
A particular VCI application involves the protection of structural cables from corrosion. Structural cables are perhaps most commonly observed in suspension and cable stay bridges. Here, they may be thousands of yards long, several feet in diameter and represent a significant long term investment. Structural cables are also a key component in a method of prestressing concrete structures, known as post-tensioning. Post-tensioned concrete systems have been used for decades in the construction of bridges, elevated concrete slabs for parking ramps and garages, and in flooring, walls and columns of commercial buildings. In this form of prestressing, cables, strands, bars, or other members of high strength steel are installed at a job site, usually housed in sheathing or tubes that prevent the steel from bonding to the concrete. After the concrete cures, the steel members are stretched by hydraulic jacks. The tensioned members act upon the concrete slab or other structure to place it in compression, considerably improving the capacity of the structure to withstand tensile and bending forces.
The term “elongate metal tension member” is used herein to refer generically to, for example, metal cables, wires, strands, bars and other elongated forms that that are used under tension to provide structural strength and/or support to another material and/or structure.
A persistent problem with elongate metal tension members is corrosion of the metal, particularly in environments involving exposure to salts and other environmental treatment materials (e.g. de-icing chemicals), acid rain, airborne salts in locations near the ocean, and high humidity. If undetected or untreated, corrosion can weaken metal tension members to the point of breakage. In typical post-tensioned structures where the cables or other members are not bonded to the surrounding concrete, breakage of a tensioned member can create a risk of serious injury and property damage. For cables and other tension members in a bridge, corrosion can weaken the integrity of the support systems leading to use restrictions, expensive repairs, premature bridge replacement, or catastrophic failure.
For post tensioned systems, a variety of solutions have been directed to the corrosion problem. For example, U.S. Pat. No. 5,840,247 (Dubois et al.) discloses a process for protecting the tendons embedded in housings by drilling holes in the housings and injecting a corrosion inhibiting liquid solution into the housings while applying a high power pulsating wave to enhance penetration.
U.S. Pat. No. 5,460,033 (VanderVelde) describes processes for corrosion evaluation and protection of unbonded cables. Holes are drilled in the concrete to expose the tendons, and a dry non-corrosive gas is passed through the conduits enclosing the tendons. The patent notes that if the evaluation of the gas indicates a humidity above sixty percent, corrosion will ensue. The humidity preferably is maintained below forty-five percent, by injection of dry nitrogen gas as needed.
U.S. Pat. No. 3,513,609 (Lang) shows tendons coated with a polymeric material such as Teflon (brand name) or an epoxy resin containing up to twenty-five percent finely ground Teflon polymer. The tendons are coated with a lubricating grease before they are covered with the plastic.
U.S. Pat. No. 4,442,021 (Burge, et al.) is drawn to a corrosion protection coating of cement containing up to ten percent corrosion inhibitors. The mixture is applied onto the metallic tendons before their enclosure.
U.S. Pat. No. 5,770,286 (Sorkin) describes a corrosion resistant retaining seal for end caps. The cap, formed of a polymeric material, contains corrosion resistant material inside the cap. The cap is intended to create a water-tight seal. The patent also describes an “ice pick” method of making a hole in the plastic sheath and injecting grease into the sleeve to displace water and prevent corrosion.
U.S. Pat. No. 5,540,030 (Morrow) describes injecting a polyurethane resin into the housing to displace water and air and prevent corrosion.
While the foregoing approaches are acceptable for a variety of applications, none of them is particularly well suited for providing corrosion protection for large scale systems in which the tension members may have considerable length, e.g. exceeding one hundred feet. Drilling holes for injecting anti-corrosive grout or oil becomes prohibitively expensive and time consuming, and corrosion of longer lengths of tensioned members is not adequately addressed by end caps or similarly restricted features. Coating tension members directly with anti-corrosive layers or films inhibits corrosion, but is not a practical approach for treating previously installed systems.
Cables used in bridges may be coated/treated at or before installation to inhibit corrosion. Further, the cables may be encased in a moisture impermeable protective sheath to further protect the cables from corrosion. However, these measures sometimes prove insufficient, and there is a need for cost effective post treatments to further inhibit corrosion.
U.S. Pat. No. 5,173,982 (Lovett et al.) describes a system for protection of cable assemblies in cable-stay bridges. Here, a corrosion resistant fluid is used to fill the space between the cable and sheath, from the top anchor (on a tower) to the lower anchor (bridge deck) for each cable. A reservoir on the top of the tower is used to fill each cable assembly. While potentially effective at reducing or eliminating corrosion, the approach has some disadvantages. First, the vertical distance from the top anchor to the bottom anchor can create significant head pressure at lower portions of the cable sheathing. Any leaks in the sheathing can result in an unintended release of corrosion inhibitor liquid into the environment, as well as loss of corrosion protection in that cable assembly. Further, on a large bridge, this may require the acquisition and handling of large quantities of corrosion inhibitor fluids.
U.S. patent application Ser. No. 11/559,482 (assigned to the present assignee) provides solution to some of the above problems. The application describes methods and systems for the prevention of corrosion, which use a powder aerosol containing volatile corrosion inhibitors. The aerosol is blown into the space between a metal tensioning element and sheath leaving powdered volatile corrosion inhibitors in place to protect the metal. However, the handling of the powder aerosol can be a concern with respect to employee safety (inhalation and explosion) as well as environmental release.
Accordingly, the present invention concerns structures, systems, and processes directed at least to one or more of the following objects:
(1) to facilitate corrosion protection of metal tension members having considerable length, without the need to drill multiple holes along the length of the members to be treated;
(2) to provide a process for treating tensioned reinforcement members in situ in preexisting structures, at low cost and minimal disruption to the structures and minimal safety and environmental risks;
(3) to provide a process particularly well suited for protecting reinforcement members (either before or after they are tensioned) enclosed in relatively tight tubes or sheaths, or having irregular or varying topographies or otherwise forming relatively small or deep voids where exposed metal surfaces are difficult to reach.